This work is concerned with the mechanisms for measuring long time intervals in the ciliate, Tetrahymena. Especially of interest is the control of maturation, after about 60 cell divisions, and of reproductive senescence, which occurs at a slow and probably constant rate in most genotypes. Cytogenetic analyses of the behavior of aged micronuclei are carried out. Efforts will be made to examine the effects of "aged" cytoplasm on micronuclear behavior, and of "immature" cytoplasm on the manifestation of macronuclear characteristics.